Author Topic: Aren't diesel engines just better?  (Read 5307 times)

May 16, 2021, 11:05:31 am


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Aren't diesel engines just better?
« on: May 16, 2021, 11:05:31 am »
Diesel lover here. I just prefer driving diesels and I'm sad to see them going. I love that torque hehe.

Can someone correct me if I'm wrong here. Isn't it true that fuel is used more efficiently in a diesel engine? As far as I understand it any fuel the engine cannot process is recycled and reused in the engine later?

For example if I'm in too high a gear, put the foot down and the engine cannot turn quick enough to process the fuel I'm injecting into it at that time, there would be surplus fuel which is then recycled into the engine later. Thus MPG isn't necessarily just affected by throttle amount. This is contrasted by petrol engines as when you accelerate more than the engine's rpm, etc can handle, fuel is wasted.

I find this stuff quite interesting, any mechanics want to correct me lol

EDIT: Thanks everyone here! I've learned a lot more specific info on this. It looks like I was vaguely thinking of EGR and it also looks like I wasn't too close with my information. I've got some homework to do 😆

Reply #1May 16, 2021, 02:56:24 pm


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Re: Aren't diesel engines just better?
« Reply #1 on: May 16, 2021, 02:56:24 pm »
Yes, some studying needs to be undertaken.  Diesel engines don't save their fuel and burn it later.  When the exhaust valve opens whatever is in there goes out.  Unless you dropped a valve and then it beats around in there pounding the head with each revolution of the crank. 

What you may be thinking of is how the diesel engine needs to spin faster to draw more fuel in.  Or have the turbo push it in.  In that regard, it is different than a gasser which may push more gas in the cylinder to make it go faster.   Before the invention and mass acceptance of fuel injectors, a gasser would have a carburetor on in and there was a fuel pump on those that squirt a stream of gas into the intake body for more acceleration.  I am not totally sure how the new gas injectors work to simulate that function.  The system is under a pressure on the fuel rails that is somewhere about 14 lbs.  If the injectors are told, electrically, that they are to stay open a tad longer they get more fuel and away you go. 

Diesel injectors are under high pressure but on IDI engines the open time is regulated by the spring pressure of the injectors.  That is the 130 or 155 bar rating on the body of them.  More pressure less time open.  Now, do the newer TDI's that have all that confusing electrical hardware attached to them act differently?  I would not be the one to ask that question as I own 1981 parts and vehicles.  I will let others with that knowledge tell you. 

Where I think types of diesel gain on efficiency over gassers is that lack of fuel entry prior to having enough air to combust it.  That and the very small amount of fuel needed to light at the top of the power stroke to send the piston back to the bottom.  The compression of air volume is so much greater in a diesel.  Gas engines can't do that because the gas is so much more volatile.  It ignites way faster than diesel.  So increasing the pressure would have it burning much earlier and the piston would be trying to go two ways, down from the burning fuel and up for the compression of the fuel/air mix. 

Not exactly a good combination.  So yeah, read up and if you find an interesting link, post it up.  There are plenty here that might like to know what you find. 

Later, and welcome.  We won't ban you for bad info.  At least we haven't done much of that. 
« Last Edit: May 16, 2021, 02:58:10 pm by ORCoaster »